Computer displays are typically driven by an analog RGB signal which is derived from a YUV to RGB conversion circuit, wherein the Y signal is well known to represent luminance (the intensity of light) of a pixel to be reproduced on the display. Unfortunately, a cathode ray tube display has an inherently nonlinear transfer function, wherein the intensity of light reproduced is proportional to the voltage input raised to the power gamma (2.5). Gamma correction is the process of compensating for this nonlinearity to achieve correct intensity reproduction.
The incoming YUV data signal can be received with or without gamma correction. The standard CCIR Rec 709 specifies the transfer function for gamma correction with exponent 0.45, wherein
R'=4.5R where R&lt;=0.018
R'=1.099R.sup.0.45 -0.099 where R&gt;0.018
and wherein
R is a normalized intensity value in the range 0 to 1 and
R' is a gamma corrected value in the range 0 to 1.
In devices used for consumer video, such as a TV display, gamma correction is done prior to transmission of the video signal, which saves the cost of adding a gamma correction circuit to every TV receiver. The standard method of obtaining the gamma correction value is by accessing a lookup table.
In a typical computer system, the color component is represented by an 8 bit digital value. For gamma correction of a YUV signal, a lookup table read only memory (ROM) of 256*8=2048 bits is required. If gamma correction is performed on an RGB signal, the system must use a ROM three times the above size, 3*2048=6144 bits. A description of gamma correction may be found in the text "Fundamentals of Interactive Computer Graphics" by JD Foley and A. Van DAM, Addison-Wesley Publishing Company, copyright 1982, pp. 564-567.